Movable contact assembly with a magnetic shield



March 3, 1964 L. A. BURTON 3,123,691

MOVABLE CONTACT ASSEMBLY WITH A MAGNETIC SHIELD Filed May 31, 1961 1 9W WW United States Patent 0 Wis.

Filed May 31, 1961, Ser. No. 113,764 5 Claims. (Cl. 20087) This invention relates to circuit interrupters. The invention particularly relates to the movable contact assembly of a circuit interrupter for a motor controller and to a device for magnetically holding the contact assembly against the force of a short circuit.

The high currents and the associated high flux density magnetic fields of a short circuit severely stress the movable contact assembly of a circuit interrupter. The force on the movable contact assembly is proportional to the square of the current, and the short circuit forces greatly exceed the normal operating forces. In some circuit interrupters, the forces of a short circuit tend to hold the contacts closed and to prevent the movable contact operating mechanism from opening the contacts. More often in a circuit interrupter these forces tend to open the contacts during a short circuit. As the contacts open, arcing occurs which may seriously damage the circuit interrupter. In an alternating current system, the contacts tend to open and close each half cycle. For the portion of each half cycle that the contacts are open, the arc heats the contacts intensely and rapidly erodes the contact surfaces. As the heated contacts reclose at each current zero, they are liable to weld together. Consequently, it is highly desirable to hold the contacts closed until they are opened by the contact operating mechanism.

Circuit interrupters that are especially intended for interrupting short circuit currents in a power transmission system usually have a mechanical latching device that holds the contacts closed until an operator releases the latch to open the contacts. In smaller circuit interrupters which are used for motor controllers, called contactors, mechanical latches have had little success. Contactors are expected to open and close a great many times under normal operating conditions, but only rarely to encounter a short circuit. It is undesirable to complicate the design and increase the cost of a contactor with latching devices, and the magnetic operator that closes the contacts remains energized to hold the contacts closed. Contactors seriously need a simple, inexpensive, hold-in device.

T he nonuniform magnetic field in the region of the current carrying movable contact accounts for the force on the movable contact assembly. The electrical system that the contactor controls forms a large loop, and the conductors of the contactor usually form several small loops that include the movable contact. The flux is ordinarily distributed nonuniformly in the neighborhood of a current loop. The prior art has suggested shaping some of the conductors to produce a contact closing force that opposes the force associated with other conductors. In many situations, however, a nonuniform field that tends to open the contacts is highly desirable because the field helps to extinguish the are that occurs as the contacts open. The magnetic field in the region of opposing contact surfaces is almost always nonuniform. The surfaces of the fixed and the movable contacts ordinarily touch only at one small point. High density current loops occur in the contacts as current flows toward this point in one contact and away from the point in the opposing contact. These currents tend to open the contacts.

There are several equivalent ways of describing the forces on a movable contact assembly in a nonuniform magnetic field. The simplest explanation for understanding this invention is that a current carrying conductor (the movable contact) tends to move in a direction to increase 3,123,691 Patented Mar. 3, 1964 the magnetic flux that links the conductor. For example, a current carrying conductor in the neighborhood of paramagnetic material tends to move toward the material regardless of the polarity of the current. The material distorts the magnetic field of the conductor, and the conductor is linked by more flux as it moves toward the higher flux density region of the material. Similarly, a current carrying loop tends to expand without regard to the polarity of the current or the magnetic field. The flux density is higher inside the loop than outside. As the loop expands, it encloses more and more magnetic flux.

The contact holding device of this invention comprises a magnetic shield that produces a local magnetic flux distribution that is contrary to the general magnetic flux distribution of the contactor. In the region of the shield the local magnetic field and the current carrying movable contact cooperate to oppose the tendency of the contacts to open or close during a short circuit. In the specific contactor that will be described, the shield is arranged with respect to the movable contact assembly to provide a closing force. An important feature of the hold-in device is that it is shaped so that a larger portion of the current carrying movable contact assembly enters a region of relatively high flux density as the contacts tend to open. The shape of the shield greatly increases the magnetic closing force as the contacts open slightly, and the shield satisfactorily limits the contact damage under even the most severe conditions.

One advantage of this contact holding device is that structural elements that are common to most contactors can be easily modified to serve also as the magnetic shield.

Another advantage of the contact holding device is that it is magnetic rather than mechanical, and it is not subjected to mechanical wear as are mechanical hold-in latches.

Another advantage of this contact holding device is that its elfect on the general flux distribution of the contactor is localized, and the general magnetic field of the contactor may have any desired shape.

One object of this invention is to provide a new and improved movable contact assembly for a contactor.

Another object of this invention is to provide a new and improved contact holding device for a contactor.

Another object of this invention is to provide a new and improved contact hold-in device that is easily adaptable to contactors of various designs.

The drawing and the description of the invention will suggest other objects and advantages of this invention.

FIG. 1 is a side view of a contactor having a contact hold-in device of this invention.

FIG. 2 is an end view of the contact hold-in device and part of the movable contact assembly of the contactor.

FIG. 3 is a section view of the contactor along line III-III of FIG. 1 and shows a map of the magnetic field in this region.

The contactor that FIG. 1 shows controls the circuit between two fixed contact assemblies 16 and 12. In the operating position of FIG. 1, a vertically movable contact assembly 14 bridges the two fixed contact assemblies and completes the circuit. In the other operating position, the movable contact assembly 14 is separated from the two fixed contact assemblies 1t and 12, and the circuit is interrupted. Each of the two fixed contact assemblies is adapted at one end to form a terminal 16 of the contactor and is provided at the other end with inserts -17 of a material that is suitable to Withstand the arcing that occurs as the contactor opens. The fixed contact assembly 12 includes a coil 19 that produces a magnetic field for blowing out the arc. In the region of the arc, the magnetic field of the coil 19 also tends to force the current carrying contacts open. The movable contact assembly 14 comprises a movable contact 23 with two contact inserts 24, an operating rod 26, a lost motion assembly 27 for mounting the movable contact on the operating rod, and a pantograph 29 for mounting the operating rod to swing the movable contact open and closed. A magnetic contact operating mechanism 36 opens and closes the contacts. The mechanism 3h comprises a stationary magnetic frame 32, a winding 34 on the frame, and an armature 35 that is connected to the pantograph 29 to operate the movable contact assembly. The contactor also includes frame members 37 that are arranged to support the elements that have been described. In addition, the contactor of FIG. 1 has an arc chute that the drawing does not show.

When the winding 34 is energized, the frame 32 attracts the armature 35 to the position that FiG. l illustrates, and the pantograph 29 raises the operating rod 26 and the movable contact assembly 14 into the closed position of FIG. 1. The winding -34 remains energized to keepthe contacts closed. When the winding 34 is deenergized, the mechanism 30 releases the operating rod 26, and the movable contact assembly 14 falls open.

The assembly 27 provides lost motion between the movable contact 23 and the operating rod 26 for properly seating the contacts. The assembly 27 comprises a spring 40, a spring saddle 42, and a contact guide 44. The guide 44 has an inverted U shape in FIG. 2, and bolts 46 secure the contact guide 44 to the upper end of the operating rod 26. The spring saddle 42 extends through an opening 47 in the movable contact 223 and the lower ends 48 of the spring saddle turn outward and engage the lower surface of the contact 23. The spring 4% is positioned between the spring saddle 42 and the upper end of the operating rod 26 and urges the movable contact 23 upward against the guide 44. The spring saddle 42 rides in the opening 49 in the spring guide 44 as the contact 23 moves relative to the operating rod 26.

The opposing contact surfaces 17 and 24 meet on a diagonal to the vertical motion of the movable contact assembly 14. Current flows somewhat parallel to the surfaces in the contacts and forms current loops that tend to open the contacts. The magnetic operating mechanism 30 has sufficient force to hold the contacts closed during any short circuit that the contactor might encounter, and the spring 40' has sufficient force to hold the contacts closed during normal operating conditions. However, the spring force is limited so that the spring does not unduly add to the force required to close the contacts and so that the spring cushions the shock of closing the contacts. The spring alone isnot sufficient to hold the contacts closed during a short circuit.

Except for features of the contact guide 44 that will be explained in more detail, the contactor as it has been described so far is typical of many well known contactors. When the contact hold-in device is explained, it will be apparent that the hold-in device can be used with other contactors of different design. For example, the invention applies also to contactors in which the movable contact pivots at one fixed terminal to connect the fixed terminal to a fixed contact.

The hold-in device includes a magnetic shield So that tends to hold the contacts closed during a short circuit. In the specific contactor of FIGS. 1 and 2, the upper portion of the contact guide 44 is of magnetic material and forms the magnetic shield 56. The lower portion 51 of the guide 44 supports the shield i) on the operating rod 26. The invention applies to contactors that differ structural ly from the contactor of FIG. 1, and the general aspects of the invention can be understood by considering the relation between the contact 23, and shield 54), and the associated magnetic field. The field map of FIG. 3 describes this relationship.

The magnetic shield 50 is U-shaped and is positioned with respect to the contact 23 to produce a local field distribution that is shown. in FIG. 3. The shield partially surrounds the conductor 23 in one direction of contact travel. The shield 5G is discontinuous and forms an air gap in the opposite direction of contact travel. The flux density is very high in the low reluctance material of the shield 56. As compared with the general field distribution of the contactor, the flux density is also quite high in the region of the portion of the conductor 23 that is in the air gap. The conductor 23 tends to move upward in the field distribution of FIG. 3 to a position where more of the high density fiux links the conductor. For the contactor of FIG. 1 the upward force of the shield 5t) coincides with upward force of the magnetic operating mechanism 34} in holding the contacts closed. For some other contactors FIG. 3 describes an upward force that helps the contact operating mechanism to open the contacts.

During a short circuit, the portions of the movable contact 23 that are outside the region of the magnetic field that FIG. 3 describes are forced downward. The opening force is particularly strong in the region of the contacting surfaces 17 and 24. In the shielded region that FIG. 3 describes, the contact 23 is forced upward. Under some short circuit conditions, the upward forces (including the force of the spring 4%) are sufficient to hold the movable contact 23 tightly closed against the two fixed contact assemblies 10 and 12. However, under severe short circuit conditions the contacts may tend to open. As the contacts open slightly, less and less of the flux in the high density region in the air gap of FIG. 3 links the conductor 23. The field in the air gap cooperates with the current carrying contact 23 to increase the closing force considerably if the contacts begin opening.

The support 51 may be of nonmagnetic material, but it is ordinarily simpler to make the shield and the support 51 of a single piece of magnetic material. If the support 51 is magnetic, it tends to shield a portion of the contact 23 from the general field of the contactor, but it does not produce a positive hold-in force. In most cases the shield 50 can be lengthened easily to provide the holding force that might otherwise be obtained by making the support 51 nonmagnetic. The opening 49 in. the shield 50 also reduces the flux that otherwise might help to hold the contact 23. It is desirable to locate the support 51 and the opening 49 so that they coincide in the magnetic circuit of the contact 23. As FIG. 1 shows, the edges of the opening 49 that parallel the flux are very nearly in line with the edges of the support 51 that parallel the flux.

When the guide 44 is a single piece of magnetic mate rial, it has a cut out 55 on either side of the support 51. The cut outs 55 reduce the width of the support 51 near the air gap of the shield 50. Thus, the cut outs 55 increase the eifective' length of the shield and increase the flux that tends to hold the contacts. The support 51 may have a somewhat wider base portion 57 for mounting the contact guide on the operating rod 26 since magnetic material in this region does not appreciably affect the fiux pattern of FIG. 3.

When the contactor is closed, the shield 50 is held securely by the operating rod 26 and by the support 51 independently of the contact 23. During a short circuit, the force that otherwise might open the contacts is transmitted magnetically through the contact guide 44 to the operating rod 26. and the magnetic operating mechanism 30. Consequently, when the magnetic operating mechanism 3t) releases the operating rod 26, the magnetic shield does not interfere with opening the contacts. In contactors of some designs, it may be preferable to mount the magnetic shield 50 on a fixed support that is mechanically isolated from the contact operating mecha nism. Since the force of the hold-in device is proportional to the square of the current, the hold-in force is normally quite small, and the hold-in device would not seriously interfere with opening the contacts under normal conditions.

Those skilled in the art will recognize that within the scope of the claims the contact hold-in device of this invention can be adapted in various forms to numerous different circuit interrupter designs.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. In a circuit interrupter in which the force on a movable contact during a short circuit opposes in one direction the force applied to the contact by a movable contact operating mechanism, a movable contact holding device comprising a U-shaped shield of magnetic material defining a magnetic circuit that includes an air gap between spaced apart ends of said shield, and means supporting said shield independently of the movable contact and positioning said shield to encircle the movable contact partially in the direction from the movable contact of the force applied to the movable contact by the contact operating mechanism and to include a portion of the movable contact outside said ends in said air gap when the contactor is closed.

2. In a contactor in which there is a force on a movable contact during a short circuit which opposes the closing force applied to the movable contact by a movable contact operating mechanism, a movable contact hold-in device comprising a U-shaped shield of magnetic material defining a magnetic circuit that includes an air gap between spaced apart ends of said shield in the direction of contact opening, and means supporting said shield independently of said movable contact to partially encircle the contact and to include a portion of the movable contact outside said ends in said air gap when the contact is closed.

3. In a contact hold-in device for a contactor in which the magnetic field distribution in the region of a movable contact tends to open the contactor, a shield of magnetic material for reversing the general flux distribution in a selected locality to oppose the forces that tend to open the contact, said shield being U-shaped to define a region of high permeability adjacent said contact in the direction of contact closing and to define an air gap between the ends of said shield in the direction of contact iii opening, and means supporting said shield against the forces of a short circuit in a position to receive said contact partly within said shield and partly within said air gap when said contact is closed.

4. In a contactor, a device for holding in a movable contact that tends to move with respect to the movable contact operating mechanism while the movable contact is carrying abnormally high current, comprising, a generally U-shaped shield of magnetic material partially oncircling said movable contact and defining a magnetic circuit with an air gap and means mounting said shield on the movable contact operating mechanism with the open end of said U-shaped shield adjacent said contact in the direction of contact closing to receive the movable contact partially within said shield and partially Within said air gap when the contactor is closed.

5, In a contactor, a device for holding in a movable contact that tends to move in the direction of contact opening with respect to the movable contact operating mechanism while the movable contact is carrying abnormally high current, comprising, a generally U-shaped contact guide of magnetic material, means mounting said contact guide on the movable contact operating mechanism with the open end of said U-shaped contact guide in the direction of contact opening, and means including a spring positioning the movable contact within said U-shaped contact guide and urging the movable contact in the direction of contact closing, said contact guide having cut out portions defining an air gap between the ends of said guide that includes a portion of the movable contact when the movable contact is closed and includes a larger portion or" the movable contact as the movable contact moves in the direction of contact opening with respect to the operating mechanism.

References Cited in the file of this patent UNITED STATES PATENTS 2,635,158 Peter Apr. 14, 1953 2,679,561 Thompson May 25, 1954 2,795,671 Edwards June 11, 1957 

1. IN A CIRCUIT INTERRUPTER IN WHICH THE FORCE ON A MOVABLE CONTACT DURING A SHORT CIRCUIT OPPOSED IN ONE DIRECTION THE FORCE APPLIED TO THE CONTACT BY A MOVABLE CONTACT OPERATING MECHANISM, A MOVABLE CONTACT HOLDING DEVICE COMPRISING A U-SHAPED SHIELD OF MAGNETIC MATERIAL DEFINING A MAGNETIC CIRCUIT THAT INCLUDES AN AIR GAP BETWEEN SPACED APART ENDS OF SAID SHIELD, AND MEANS SUPPORTING SAID SHIELD INDEPENDENTLY OF THE MOVABLE CONTACT AND POSITIONING SAID SHIELD TO ENCIRCLE THE MOVABLE CONTACT PARTIALLY IN THE DIRECTION FROM THE MOVABLE CONTACT OF THE FORCE APPLIED TO THE MOVABLE CONTACT BY THE CONTACT OPERATING MECHANISM AND TO INCLUDE A PORTION OF THE MOVABLE CONTACT OUTSIDE SAID ENDS IN SAID AIR GAP WHEN THE CONTACTOR IS CLOSED. 